1. 9/12/2012PHY 113 A Fall 2012 -- Lecture 71 PHY 113 A General Physics I 9-9:50 AM MWF Olin 101 Plan for Lecture 7: Chapter 5 – NEWTON’S GENIUS 1.Concept of force 2.Relationship between force and acceleration

2. 9/12/2012 PHY 113 A Fall 2012 -- Lecture 72

3. 9/12/2012PHY 711 Fall 2012 -- Lecture 73

4. 9/12/2012PHY 711 Fall 2012 -- Lecture 74

5. 9/12/2012PHY 113 A Fall 2012 -- Lecture 75 Isaac Newton, English physicist and mathematician (1642—1727) http://www.newton.ac.uk/newton.html 1.In the absence of a net force, an object remains at constant velocity or at rest. 2.In the presence of a net force F, the motion of an object of mass m is described by the form F=ma. 3.F 12 =– F 21.

6. 9/12/2012PHY 113 A Fall 2012 -- Lecture 76 Detail: “Inertial” frame of reference Strictly speaking, Newton’s laws work only in a reference frame (coordinate system) that is stationary or moving at constant velocity. In a non-inertial (accelerating) reference frame, there are some extra contributions. iclicker question: Are we (here in Winston-Salem) A.In an inertial frame of reference (exactly) B.In a non-inertial frame of reference Newton’s laws are only approximately true in Winston- Salem, but the corrections are very small.

7. 9/12/2012PHY 113 A Fall 2012 -- Lecture 77

8. 9/12/2012PHY 113 A Fall 2012 -- Lecture 78 Example force – weight 1 lb = 4.45 N 1 N = 0.225 lb Newton

9. 9/12/2012PHY 113 A Fall 2012 -- Lecture 79 Gravitational mass versus inertial mass – The concept of mass (“inertial” mass m) is distinct from the concept of weight mg. In fact the gravitation “constant” g varies slightly throughout the globe. Of course the weight of mass m on the surface of the moon is considerable different than its weight on earth.

10. 9/12/2012PHY 113 A Fall 2012 -- Lecture 710 Isaac Newton, English physicist and mathematician (1642—1727) http://www.newton.ac.uk/newton.html 1.In the absence of a net force, an object remains at constant velocity or at rest. 2.In the presence of a net force F, the motion of an object of mass m is described by the form F=ma. 3.F 12 =– F 21.

11. 9/12/2012PHY 113 A Fall 2012 -- Lecture 711 Examples F gravity =-mg j F tension = T j F=F net =F tension +F gravity =(T-mg) j =0

12. 9/12/2012PHY 113 A Fall 2012 -- Lecture 712 Examples F gravity =-mg j F tension = T j F=F net =F tension +F gravity =(T-mg) j =0 F gravity =-mg j F=F net =F gravity =(-mg) j = ma a=-gj

13. 9/12/2012PHY 113 A Fall 2012 -- Lecture 713 iclicker question: An object having a mass of 2 kg is moving at constant velocity of 3 m/s in the upward direction near the surface of the Earth. What is the net force acting on the object? A.0 N. B.6 N in the upward direction. C.25.6 N in the upward direction. D.19.6 N in the downward direction.

14. 9/12/2012PHY 113 A Fall 2012 -- Lecture 714 Example: (assume surface is frictionless) Suppose T measured a observed & measured T = m a

15. 9/12/2012PHY 113 A Fall 2012 -- Lecture 715 Example – support forces  F support acts in direction  to surface (in direction of surface “normal”)

16. 9/12/2012PHY 113 A Fall 2012 -- Lecture 716 Newton’s second law F = m a Types of forces: Fundamental Approximate Empirical Gravitational F=-mg j Friction Electrical Support Magnetic Elastic Elementary particles

17. 9/12/2012PHY 113 A Fall 2012 -- Lecture 717 Examples (one dimension): (constant)

18. 9/12/2012PHY 113 A Fall 2012 -- Lecture 718 Newton’s third law F 12 = - F 21 12 FPFP T T T=m 1 a a=F P /(m 1 +m 2 ) F P -T=m 2 a T=F P (m 1 /(m 1 +m 2 ))

19. 9/12/2012PHY 113 A Fall 2012 -- Lecture 719 T-m 1 g=m 1 a T-m 2 g=-m 2 a => after some algebra:

20. 9/12/2012PHY 113 A Fall 2012 -- Lecture 720 F gravity =-mg j F scale F scale + F gravity = 0; F scale = mg Question: What if you step on the scale in the elevator? F scale + F gravity = m a; F scale = mg +m a

21. 9/12/2012PHY 113 A Fall 2012 -- Lecture 721

22. 9/12/2012PHY 113 A Fall 2012 -- Lecture 722 mg F floor = mg mg F floor = mg-T T T Mg

23. 9/12/2012PHY 113 A Fall 2012 -- Lecture 723 Exercise For this exercise, you should ride the elevator from the first floor of Olin Physical Laboratory up to the second or third floor. While in the elevator, stand on the scale and record the scale readings to answer the following questions. 1. What is the scale reading when the elevator is stationary? 2. What is the scale reading when the elevator just starts moving upward? 3. What is the scale reading when the elevator is coming to a stop just before your destination floor? x <x >x